compression basics pls?

[26667]

I have a very rudimentary understanding of the why and wherefore of a compression test. It's one of the few things left on our to-do list. Anybody have time to explain a bit to me? i.e. how to, what if it's low? different on each side?what to do etc.

we won't do it till I have time to get back to the guru's shop, but I'm learning...slowly.
thanx.

 

[pglaves]

I have a very rudimentary understanding of the why and wherefore of a compression test. It's one of the few things left on our to-do list. Anybody have time to explain a bit to me? i.e. how to, what if it's low? different on each side?what to do etc.

we won't do it till I have time to get back to the guru's shop, but I'm learning...slowly.
thanx.

The test is simple - what to do afterwards depends.

To get an accurate test you have to get air into the cylinder in order to compress it. So the proper procedure is to do the test with the carburetors disconnected from the heads. This is so the throttle plates and slides are not blocking airflow in.

Warm the engine to operating temperature. Remove the spark plugs. Insert the plugs back into the wires and use jumper wires to ground the spark plugs. This will avoid potential damage to the ignition system.

There are generally two types of compression testers. Some have conical rubber tips that are tightly pressed into the spark plug hole while cranking. Others have a threaded connector on a hose that screw into the spark plug hole and seal with an O ring.

In either case - affix the tester to the spark plug hole and crank the engine. Let it turn over several times. Read the compression - it is normally in p.s.i. - pounds per squire inch.

Compare that value to your manual.

My Haynes manual for Airheads says the following:

below 109 p.s.i. = poor
109-131 = normal
over 131 = good

Exactly why "normal" isn't good escapes me, but that's Haynes.

 

[skiteach]

It's also used as a comparison between cylinders. They should be similar.

 

[pglaves]

It's also used as a comparison between cylinders. They should be similar.

Of course.

 

[26667]

thanx and...

thanx, you guys, and then what is that causes the compression to drop? Air leaks? If so, could it be something as simple as the black rubber air tubes; their integrity and quality of their seals? Improper valve seating at some point of the cycle?

 

[pglaves]

thanx, you guys, and then what is that causes the compression to drop? Air leaks? If so, could it be something as simple as the black rubber air tubes; their integrity and quality of their seals? Improper valve seating at some point of the cycle?

You are thinking more of air induction.

Compression is the result of the piston moving from the bottom of the stroke to the top of the stroke - with the intake and exhaust valves closed. It "compresses" all of the air in the cylinder up into the small pocket in the head - combustion chamber.

Compression "leaks" past rings and valves.

 

[sumran]

Exactly why "normal" isn't good escapes me...

Perhaps they have low expectations.

 

[108625]

thanx, you guys, and then what is that causes the compression to drop? Air leaks? If so, could it be something as simple as the black rubber air tubes; their integrity and quality of their seals? Improper valve seating at some point of the cycle?

What you need to answer that question is a leakdown test, a whole different animal (and thread subject).

 

[26667]

aha!!

I get it....mostly. Thanx to all above......not god, the guys who wrote above this post

 

[26667]

Fwiw2u

Interesting procedure. And bothe sides at 175psi.
I guess we'll continue to search for the cause of this annoying vibration in the pegs. Sumpin' just ain't quite right.

 

[Geoff]

The test is simple - what to do afterwards depends.

To get an accurate test you have to get air into the cylinder in order to compress it. So the proper procedure is to do the test with the carburetors disconnected from the heads. This is so the throttle plates and slides are not blocking airflow in.

Warm the engine to operating temperature. Remove the spark plugs. Insert the plugs back into the wires and use jumper wires to ground the spark plugs. This will avoid potential damage to the ignition system.

There are generally two types of compression testers. Some have conical rubber tips that are tightly pressed into the spark plug hole while cranking. Others have a threaded connector on a hose that screw into the spark plug hole and seal with an O ring.

In either case - affix the tester to the spark plug hole and crank the engine. Let it turn over several times. Read the compression - it is normally in p.s.i. - pounds per squire inch.

Compare that value to your manual.

My Haynes manual for Airheads says the following:

below 109 p.s.i. = poor
109-131 = normal
over 131 = good

Exactly why "normal" isn't good escapes me, but that's Haynes.

Not being a smartass Paul, but how do you get the engine up to operating temp with the carbs disconnected? Ah, nevermind. I just looked at my bike and see it's a simple matter of loosening a couple of hose clamps. Never mind...

 

[AnnapolisAirhead]

Interesting procedure. And bothe sides at 175psi.
I guess we'll continue to search for the cause of this annoying vibration in the pegs. Sumpin' just ain't quite right.

Have you had the carbs properly balanced (mixture too)? These things are awfully darned sensitive to balance and cause vibration at initial acceleration and/or at speed. There are a few ways to balance them. That's one of the most endearing things I learned how to do--made my ride much more enjoyable.

When does it vibrate (I mean besides always...it IS a 2 banger motorbike)? At idle? Acceleration? What speed and what RPM?

It's real easy to 'lug' these bikes if you are new to them...they don't like it. Probably not good for them either. So what RPMs are you shifting the bike? Cruising (gear and RPM)?

 

[26667]

Thanx, Double A, but we've balanced and synched the carbs, adjusted the valves and triple checked the work. It idles beautifully, but the pegs start buzzing shortly after 2000 rpm. They're at their worst btw 3500 and 4200, in any gear, and then sort of lighten in intensity after 4500. But the speed of the vibe increases w the engine speed right on thru 5000rpm. I've ridden airheads for almost 25 years now and I know that sweet spot at about 4200. It's just not there, and this vibration is not "normal.''

i'm headed to Chi BMW tomorrow. One of the airhead experts there has agreed to ride it and see what he thinks.

 

[sumran]

Did you have this problem before you did your top end work? You could have timing or ignition issues.

It could be caused balancing problems. I don't remember; did you do a complete top end or just the heads? Did you check balance on new parts? Have you done any clutch work?

Thanks for keeping us informed. Hope you solve it quickly and inexpensively.

 

[26667]

Thanx, Sum'. We haven't checked the timing since the first 500 miles of the top-end. I expect to learn more or at least get a new idea in a few hours.

I'd have to look at the receipt to tell you exactly what was done, but if memory serves, valves, guides and seats. Randy Long, who did the work thought the rest looked good.

The break-in went as by-the-book and the bike was starting to really run good and strong till the 1500 mi marker on the new valves. Then when I re-torqued the heads and adjusted the valves, as per the manual, I also replaced the lowers. Had to remove and replace the exhaust to do it. First I thought I'd left something loose or askew on the exhaust so I took it apart and re-did it. Nope. then I re- adjusted the valves. Nope. The carbs seemed good, it idled great. So I re-checked the valves. The R exhaust had a slightly ezier pull so I tightened it to a slightly firmer pull. Nope. Then guru Seymour re-checked the carbs....and re-checked the valves. Perfect. Result: head scratching and peg vibrating.

The only other thing I notice is that it doesn't quite have the quick acceleration that had sort of startled me when I first got the machine. I hadn't realized what a difference there was btw this older RS, and the '82 R100T I'd owned for twenty years. I'm sure someone knows whether it's a different cam or differen gear ratio or what, but this one, when it's right, goes like a scalded ape.

 

[LOSTBOY]

Compresion testing

When you R&Red the exhaust system, did you by chance swap the hanger/clamps at the motor mount bolts? They are "handed" left and right. Dd you torque the motor mount nuts?

 

[26667]

Thanx. LB. Tho' I had a shop in LA CA do that to me once...I thought, "how the hell did my mufflers get so cruddy on top??" Then realized the shop had put them on upside-down. Sheesh.

But these are on correctly. I only pulled the header pipes far enough to re-install the lowers.

We're now thinking that the new throttle cables aren't balanced at so-called "high idle" and need to be balanced at about 2-3000 rpm, as well as at idle. It makes sense anyway. It idles beautifully at several idle speeds, btw 900 and 1100, (We've balanced those carbs at least a dozen times) but if one cable's pulling harder than the other, that'd be a likely culprit for both the vibe and the slight lack of go-fast.

So maybe monday or tuesday....I think the universe is trying to teach me patience. I can't wait till I get it.

 

[991100s]

Check to see if a wheel is draging. You could have a piston in a caliper that is sticking.